Recently, with the developments in electronic instruments, there has been a demand for development of compact, lightweight, high-energy-density, rechargeable electrochemical devices. In particular, there has been an expansion in the use of high-energy-density lithium-ion secondary batteries and long-life, high-current-producible electric double layer capacitors and redox capacitors, because of their advantages.
Recently, there has been a drastic increase in the demand for electric double layer capacitors serving as memory-backup power sources, which use an electric double layer formed on an interface between a polarized electrode and an electrolyte. Attention for electric double layer capacitors has also been focused on large-capacitance-demanding applications such as electric vehicle power sources.
In the fields of cellular phones, laptop computers and the like, there has been a sharp increase in the demand for lithium-ion secondary batteries using LiCoO2, LiNiO2, LiMn2O4, or the like as a positive electrode active material and graphite or the like as a negative electrode active material, because of their performance. In addition, attention has been focused on the capacity size of redox capacitors using oxidation-reduction reaction (pseudo-capacitance of electric double layer) on the surface of metal oxides or electrically conductive polymers.
These electrochemical devices have been practically developed for the last dozen years or so, and many patent applications and literatures on these techniques have been published in recent years. For example, methods of manufacturing electrodes for electric double layer capacitors are proposed which include kneading activated carbon powder with a solvent of a liquid electrolyte such as sulfuric acid to form a slurry and forming the slurry by pressing (U.S. Pat. No. 3,288,641). However, the electrode formed by this method has a rigid porous structure and can easily crack or collapse. Thus, it cannot endure long-term use.
In order to produce electrodes having anti-cracking or anti-collapsing properties and good form retention, it is proposed that the method of manufacturing electrodes should include preforming a kneaded product of a carbonaceous material such as activated carbon, a binder such as polytetrafluoroethylene (PTFE) and a liquid lubricant and then drawing or rolling the preform into a sheet-shaped product (Japanese Patent Application Laid-Open (JP-A) No. S63-107011 and JP-A No. H02-235320).
In this method including the kneading step, however, PTFE can be partially formed into fibers and partially not formed into fibers. In the process of forming a thin film-shaped electrode sheet, therefore, uneven surfaces can be easily formed, and thus performance of the resulting electrochemical device can be unsatisfactory.
There is also proposed a method including the steps of mixing activated carbon powder, PTFE and a solvent to form a paste, applying the paste to a collector, drying it, then heating it to the melting point of PTFE or higher, and press-forming it to form a thin-film electrode and to increase its density (JP-A No. H09-36005). However, this method has complicated processes.
There is also proposed a method including the steps of mixing activated carbon powder, an aqueous dispersion of a styrene/butadiene polymer and a water-soluble thickening binder to form a paste, applying the paste to a collector, drying it, and then press-forming it to form a thin-film electrode and to increase its density (JP-A No. H11-162794). In this method, however, the viscosity of the slurry highly depends on the solid content, and thus a slight increase in slurry concentration can lead to a great change in slurry viscosity. Therefore, the coating performance can easily be degraded, and continuous production is not possible by this method.
As mentioned above, it is difficult to achieve continuous production by any of the conventional manufacturing methods, and for example, a long electrode sheet cannot be efficiently produced. The conventional methods have a problem with mass production.
The invention has been made in order to solve the problems with the above prior art, and it is an object of the invention to provide a method of manufacturing an electrode for electrochemical device suitable for industrial-scale mass production.